1. Field of the Invention
The present invention relates to an engine of such a type that a piston is reciprocatingly driven by fluid pressure such as pneumatic or hydraulic pressure.
2. Prior Art
The present invention provides an art that is an improvement of such an apparatus as disclosed in U.S. Pat. No. 4,812,109 (Yonezawa) one of the present inventors has previously proposed.
The following description is directed to objects of the improvement in the above prior art, and more particularly to the constructions and problems thereof. Throughout this description of the prior art, component members which correspond to their counterparts in FIG. 1 of the above-mentioned U.S. Pat. No. 4,812,109 are referenced by the same numerals, here parenthesized, as in the FIG. 1.
The prior art mentioned above is so constructed that the pilot valve casing of a pilot valve (18) is secured at its upper portion to a supply-discharge valve casing (29) and that a pilot valve seat (48) formed of an O-ring is received by the lower portion of a support cylinder (49) provided downwardly protrusively from the pilot valve casing.
The conventional construction as shown above is advantageous in that in the descending process of the supply-discharge valve member (30), the back pressure of the valve member (30) can rapidly be reduced on its way of descent, whereby the engine can be prevented from being stopped in operation even when the piston (8) is driven at a very low speed. However, there has been a problem that the engine may be stopped operating when the piston (8) is driven at a still lower speed than above.
This problem can be attributed to the following reason:
When the piston (8) is actuated to descend toward its bottom dead point at an extremely low speed, the resulting speed at which a pilot valve member (46) secured to the piston (8) leaves apart from the pilot valve seat (48) also becomes lower to a great extent. Due to this, the pressure fluid drawn into a discharge actuation chamber (35) from a supply actuation chamber (33) is reduced in pressure through a narrow clearance between the valve member (46) and the valve seat (48), pressurizing the inside of the discharge actuation chamber (35) only at an extremely low speed. This causes the descending speed of the supply-discharge valve member (30) to become lower in the first half of its descent because of a small amount of force for pushing down the valve member when it has started to descend. As a result, the pressure fluid in the fluid pressure actuation chamber (9) is permitted to escape from an operation chamber (32) to a discharge chamber (34) before the back pressure of the supply-discharge valve member (30) turns to a smaller value, thus the supply-discharge valve member (30) being stopped on its way of descent, with the engine stopped operating.